1. Field of the Invention
This invention relates to a fuel supply system for a direct fuel injected engine. More particularly, the present invention relates to an improved fuel supply system that is most suitable for direct fuel injected engines used outboard motors.
2. Related Art
In all fields of engine design, there is a demand for obtaining more effective emission control and better fuel economy while at the same time increasing power output. To meet this demand, fuel injection systems have replaced carburetors as the engine charge former. In such systems, fuel is typically injected into an intake air manifold. In order to achieve even better performance, direct fuel injections systems have been developed. These systems inject fuel directly into the combustion chamber through a fuel injector. The principal advantage of direct fuel injection systems is that mixing of the fuel and the air within the combustion chamber can be precisely controlled.
To further improve performance, direct fuel injection engines typically include an air/fuel ratio sensor for detecting the air/fuel ratio in the combusted exhaust gases. This information is used by an engine control system to adjust the amount of fuel injected into the combustion chamber. In a fuel injected engine, the amount of fuel being injected into the engine is typically calculated from the fuel pressure at the fuel injectors and the duration that the fuel injectors are opened. Accordingly, fuel injected engines often include a fuel pressure sensor for calculating the amount of fuel injected into the combustion chamber.
There are several problems associated with calculating the fuel pressure. For example, because direct fuel injection engines typically require a high fuel pressure, the fuel pipes of the fuel system are typically made of metal. Pressure pulsations caused by a fuel pump are amplified by these metal components. This reduces the accuracy of the fuel pressure measurement, which can result in an inaccurate amount of fuel being injected into the combustion chamber. This can impair the emissions, fuel economy and power of the engine.
The pressure fluctuations in the fuel system can be reduced, to an extent, by a pressure regulator. However, the fuel injectors are often located downstream from the pressure regulator. For example, in outboard motors, the crank shaft is disposed vertically. Typically, fuel is supplied to the fuel injectors through fuel rails, which extend vertically from a high pressure fuel pump located above the fuel injectors. The pressure regulator typically is also located above the fuel injectors. With this arrangement, the pressure regulator is located a significant distance from the fuel injectors. Accordingly, pressure fluctuations at the fuel injectors are particularly large especially for the fuel injectors located at the end of the fuel rail farthest from the pressure regulator.
Another problem associated with calculating fuel pressure is that the fuel pressure sensors themselves often produce inaccurate measurements. Fuel pressure sensors typically include fine distortion gauges and circuits that are easily damaged, especially by excessive vibration. However, the fuel pressure sensors are typically directly attached to the fuel system, which is mostly made of metal components that effectively transmit the vibrations produced by the engine. Accordingly, it is difficult to prevent the fuel pressure sensor from being damaged by the engine vibrations. This can also result in an inaccurate amount of fuel being injected into the combustion chamber.